In an injection molding field, when mold closing operation in mold clamping process is completed, a mold clamping force durable to the entire injection processes including a hold pressure process is applied to a mold, and thereafter, a molten resin injecting operation is started. As a result, a movable side mold and a stationary side mold are kept firmly pressed against each other at the initial injection stage, so that gas breathing from a mold cavity becomes poor. This sometimes entails the hindrance to a filling operation of the molten resin or a molding failure such as a burn of the molding. Also, since it is necessary to start the injection operation after completing the mold clamping operation, the speeding of the molding cycle has been limited. Such a problem caused by the gas breathing fault can generally be solved by providing a gas vent in a mold. However, there is a dilemma as follows; if the gas vent is carelessly provided in the mold, burrs may be formed when a high holding pressure is applied to the mold, and, in the case where a wall thickness of the molding products is relatively large, a sink mark may occur unless a holding pressure is applied.